Submerged aqua-culture nets have a tendency to grow marine fouling algae and other contaminations which block the mesh forming the nets, thereby restricting water flow and eventually preventing any water exchange at all, across the nets. Anti-foul paint and other methods are frequently used on such net meshes in order to slow the rate of contamination accumulation; however, cleaning is always required as the anti-fouling methodologies to date do not provide a complete prevention of the accumulation of marine growth fouling such submerged nets.
In order to clean submerged aqua-culture nets, one or two methodologies have historically been adopted, including the physical removal of the nets from their in situ position, where they are taken ashore and physically cleaned; or, the use of divers sweeping a jet of high pressure water across the nets in order to blast the fouling marine growth from the nets remaining in situ.
Both the above methods of cleaning nets are highly unsatisfactory with a task of physically removing the nets from the submerged position being a huge task requiring very large lifting/handling gear and transportation of the removed nets on boats with the fouled nets weighing as much as 20 tons. Following transportation to shore the nets have to be unwound and physically cleaned and subsequently returned and reinstalled. The alternative, in situ method of cleaning using divers involves a high level of danger and risk to the operators in question, with the divers having to operate within the confines of submersion using a jet of high pressure water swept across the net so as to blast the marine growth from the surface of the net. Whilst the use of divers alleviates the problem of physically removing the nets from the water, it is still an expensive, slow and imperfect solution to the problem with four divers achieving an average of 2500 square meters cleaning per day, being the equivalent to the base and sides of one net alone. Another problem with the use of divers blasting the growth from the nets, is the dislodgement of weed and other marine organisms which drift through the cage of fish, thereby lowering the water quality and stressing the fish within a given cage. As an adaptation of the diving methodology, vacuum cleaning head can be used by divers, which extends the life of the anti-foulant coating as it does not move so much of the anti-foulant during the blasting process and also allows the marine growth and contaminants to be pumped out of the cage environment rather than drifting through the environment. In addition, the vacuuming process as manually applied by the divers is found to be faster than blasting, with four divers being able to achieve approximately 3500 square meters per day.
Nevertheless, the use of divers for cleaning of submerged nets is highly labour intensive, slow and of high risk as it involves divers being in the water for long periods of time and requires a lot of repetitive strenuous movement with the arms and shoulders to remove the hose and suction head around the net as required. The likelihood of decompression illness (bends) is much greater with exertion while diving, making net cleaning a particularly high risk dive activity.
Diving is also an inherently inefficient way of working because of the necessity of having two divers working underwater and two working on the surface at any one given time. In addition, divers are paid for their skills and tend to be more expensive than other staff, with each farming site requiring one dive crew who are primarily required to perform diving for purposes other than cleaning of the submerged nets. Accordingly each dive crew does not have enough “spare” diving time to wash up two nets per day, which is needed to keep up with the average rate of fouling. Most farming sites accordingly manage with the compromised and imperfect cleaning solutions described above, which invariably leads to nets having a greater degree of fouling than is desirable.
One object of the invention is to provide an improved submerged aqua-culture net cleaner.